A conventional X-ray tube will be explained below with reference to FIGS. 2A and 2B by taking, as an example, an X-ray tube used in an X-ray diffraction apparatus described in Jpn. Pat. Appln. KOKAI Publication No. 2006-278216. Note that FIG. 2B shows the interior of the X-ray tube viewed along line B-B in FIG. 2A.
An X-ray tube 1 includes a vacuum enclosure 2 having a vacuum interior. The vacuum enclosure 2 is obtained by connecting an insulating enclosure 3 at one end to a metal enclosure 4 at the other end. The X-ray tube 1 also includes a cathode electron gun 6 having a filament coil 5, an anode 7, and X-ray radiation windows 8a and 8b. 
The cathode electron gun 6 is arranged in the vacuum enclosure 2, and supported by the insulating enclosure 3. The filament coil 5 is centered around an X-ray tube central axis O such that the longitudinal direction is a direction perpendicular to the X-ray tube central axis O.
The anode 7 is supported by the metal enclosure 4. The anode 7 is placed in a position on the X-ray tube central axis O where the anode 7 faces the filament coil 5. A focal point 9 is formed into a rectangle on the anode 7. That is, the focal point 9 is formed on the anode 7 when electrons emitted from the filament coil 5 are converged into a rectangular electron beam having a long side in the longitudinal direction of the filament coil 5. Note that the focal point 9 is called a point focus when viewed from the short side of the rectangle, and called a line focus when viewed from the long side of the rectangle.
The X-ray radiation windows 8a are formed in the circumferential wall of the metal enclosure 4 in the direction perpendicular to the X-ray tube central axis O and in a direction perpendicular to the longitudinal direction of the filament coil 5. The X-ray radiation windows 8a extract, outside the metal enclosure 4, X-rays emitted in the direction perpendicular to the longitudinal direction of the filament coil 5.
The X-ray radiation windows 8b are formed in the circumferential wall of the metal enclosure 4 in the longitudinal direction of the filament coil 5. The X-ray radiation windows 8b extract, outside the metal enclosure 4, X-rays emitted in the longitudinal direction of the filament coil 5.
As shown in FIG. 2B, the outer circumferential shape of the cathode electron gun 6 is a circle. Also, the outer circumferential shape of the metal enclosure 4 having a restricted electrical insulation distance to the outer circumferential surface of the cathode electron gun 6 is a circle. The X-ray radiation windows 8a on the line-focus side and the X-ray radiation windows 8b on the point-focus side are arranged in the circumferential wall of the circular metal enclosure 4. Therefore, the distance from the X-ray tube central axis O to the X-ray radiation windows 8a on the line-focus side is the same as that from the X-ray tube central axis O to the X-ray tube radiation windows 8b on the point-focus side. Note that the center of the focal point 9 as the X-ray generation source of the anode 7 is positioned on the X-ray tube central axis O.
In an X-ray diffraction apparatus and the like, an optical element for collecting X-rays radiated from the X-ray tube 1 is placed at or outside the X-ray radiation window 8a. To increase the collection efficiency, this optical element is preferably placed as close as possible to the focal point 9 of the anode 7.
When using X-rays on the line-focus side, however, the distance from the focal point 9 (X-ray tube central axis O) of the anode 7 to the X-ray radiation window 8a on the line-focus side is relatively great. When the distance from the focal point 9 to the X-ray radiation window 8a is great, it is impossible to well increase the collection efficiency of the optical element.